1. Field of the Invention
The present invention relates to liquid crystal displays using vertical alignment and compensation films.
2. Description of the Related Art
A liquid crystal display has two substrates opposite each other and a liquid crystal layer interposed between the substrates. If the electric field is applied to the liquid crystal layer, the liquid crystal molecules changes their orientations to control the transmittance of the incident light.
A twisted nematic (TN) liquid crystal display includes a couple of transparent substrates having transparent electrodes thereon, a liquid crystal layer interposed between the substrates, and a couple of polarizers which are attached to the outer surfaces of the substrates. In off state, i.e., in absence of the electric field, the molecular axes of the liquid crystal molecules are aligned parallel to the substrates and twisted spirally by a constant pitch from one substrate to the other substrate, and the director of the liquid crystal layer varies continuously.
However, the contrast ratio of the conventional TN mode liquid crystal display, especially in normally black mode, may not be so high because the incident light is not fully blocked in absence of the electric field.
To solve this problem a vertically aligned twisted nematic (VATN) mode liquid crystal display is proposed in the U.S. Pat. No. 5,477,358, “CHIRAL NEMATIC LIQUID CRYSTAL DISPLAY WITH HOMEOTROPIC ALIGNMENT AND NEGATIVE DIELECTRIC ANISOTROPY”, whose patentee is Case Western Reserve University and in “Eurodisplay '93”, pp. 158-159 by Takahashi et al.
On the contrary to the TN mode, the alignment of the liquid crystal molecules of the VATN mode liquid crystal display in off state is similar to that of the TN mode in on state, that is, the liquid crystal molecules align perpendicular to the substrates. In the on state, the molecular axes of the liquid crystal molecules are aligned parallel to the substrates and twisted spirally by a constant pitch from one substrate to the other substrate, and the director of the liquid crystal layer varies continuously.
In case of VATN mode liquid crystal display in normally black mode, sufficient darkness in off state because the molecular axes of the liquid crystal molecules are aligned vertically to the substrate when the electric field is applied.
The display characteristics of VATN may become better by optimizing the parameters such as the ratio d/p of the cell gap d to the pitch p of the liquid crystal layer, the difference of refractive indices Δn between in both directions and the retardation value Δn*d.
In the mean time, because of the refractive anisotropy of the liquid crystal material the retardation value Δn*d changes as the viewing direction, thereby causing the difference of the intensity and the characteristics of light. Therefore, TN displays have the change in contrast ratio, color shift, gray inversion, etc. according to the viewing angle.
TN LCDs with compensation film are developed to compensate the difference of retardation in liquid crystal layer. However, the film compensated TN LCDs still have problems such as in harmony of the display characteristics and gray inversion.